You have indeed realistic chances to obtain Bulgarian citizenship by origin. The hard part will be getting Bulgarian citizenship for your father first, based on the fact that his mother is Bulgarian national. Once this is achieved, you will be able to apply for your citizenship based on your father's status. You will not have to surrender your U.S. citizenship, nor do you need to pass any exams (language, history, etc.).
How can I obtain Bulgarian citizenship based on family roots?
My father is a Russian citizen who was born in Varna, Bulgaria. His mother is a Bulgarian citizen. Do I have a chance to get a Bulgarian citizenship base on my family roots? I am a citizen of the U.S.